1. Field of the Invention
The present invention relates to semiconductor acceleration sensors and their production method. More specifically, the present invention relates to semiconductor acceleration sensors to be applied to automobiles and the like and their production.
2. Description of Prior Art
FIG. 7 shows a lateral cross section of a conventional semiconductor acceleration sensor, where a semiconductor acceleration sensor element 4 includes an acceleration detection beam with a thin portion and a gauge resistance (not illustrated) arranged in the form of a bridge circuit. The beam is fixed to a base 1 with a pedestal 3 to make it a cantilever. The gauge resistance is electrically connected to an external terminal 2 with a thin metallic wire 5. The semiconductor acceleration sensor element 4 and other elements described are covered with a cap 7, and are attached to the base 1 by welding.
The space between the base 1 and the cap 7 is filled with oil, for example, silicone oil 9. The silicone oil 9 is a damping fluid that prevents possible damage of the semiconductor acceleration sensor element 4 owing to strong vibration and false responses due to extraneous vibrations. When the acceleration of an automobile is detected by a conventional semiconductor acceleration sensor, the ambient temperature will range from -40.degree. C. to +120.degree. C. Therefore a sponge is arranged within the package as a pressure cushioning material to prevent possible damage due to expansion or contraction of the silicone oil 9 as a damping fluid. In the figure, a sponge 6 is fixed onto the top of the cap 7 with an adhesive 8.
When accelerated, the semiconductor acceleration sensor element 4 deflects and stress is concentrated on its thin portion. This stress is converted into an electrical signal by a bridge circuit formed by the gauge resistors and detected as acceleration.
The semiconductor acceleration sensor is used in a wide temperature range as described above. Leaving a semiconductor acceleration sensor in a severe service temperature, especially at a low temperature, will cause air bubbles to form in the silicone oil 9. The difference between the coefficient of thermal expansion of the base 1 and said cap 8, and that of said silicone oil 9 will cause the silicone oil 9 to contract at a low temperature, and part of the inside of the package will become vacuum, generating air bubbles in the silicone oil 9.
In the conventional semiconductor acceleration sensor mentioned above, air bubbles generated from the silicone oil 9 may adhere to the semiconductor acceleration sensor element 4, resulting in non-linearities in the characteristics of the semiconductor acceleration sensor and possible generation of false signals.